In British Patent Application No. 40412/77 corresponding to which is a U.S. patent application Ser. No. 943,693 of Roderick Iain Davidson, filed Sept. 19th 1978, now U.S. Pat. No. 4,275,294, there is disclosed a security system comprising fibre-optic wave guide means disposed to extend along a boundary, an energy source positioned to direct optical radiation along said guide means from one end, and a detector positioned to detect radiation leaving said guide means from its other end and arranged to change a state of said detector in response to any disturbances to said guide means other than negligible disturbances. The wave guide means can, for example, be installed in a wall or mounted in conduit disposed along the boundary in question. In a preferred security system, however, disclosed in the aforementioned British and U.S. patent applications, there is used an elongate body of the kind specified above which is in the form of a strip and includes a single fibre-optic filament. The strip is incorporated in a fence running along a boundary. Light is directed into one end of the fibre-optic filament and changes in detected light intensity at the other end of the filament, caused by significant movement of the strip or the strip being cut, are used to detect any attempt, even unsuccessful, to cross the boundary. Such changes in detected light intensity are used to operate an audible or visual alarm.
The aforementioned British and U.S. patent applications disclose a method of manufacturing the strip in which a fibre-optic filament is positioned against a core, having the form of a strip so as to extend longitudinally of the core, and then the core and the introduced filament are coated with material. In a typical example, the core is a steel strip and the coating material is zinc which is applied to the steel strip and fibre-optic filament by immersion in a molten zinc bath. For practical reasons, this method of manufacture is not always wholly satisfactory.